Usually, control rods for boiling water reactors (BWR) comprise a central part and four control rod blades protruding from the central part, each provided at a right angle in relation to the adjacent control rod blades. Usually, conventional control rod blades are manufactured of a sheet of a steel material. The control rod blades are at a free edge portion provided with a longitudinally milled recess. A plurality of channels is drilled in the recess in a row above each other. The channels have a diameter, which is less than the thickness of the metal sheet such that a satisfactory wall thickness is obtained. The channels extend from the free edge portion of the control rod blade towards the central part of the control rod. Thereafter, a suitable absorber material is applied in the channels. The absorber material may be powdered or in the form of pins. The absorber material is enclosed by rolling the legs of the recess together. Thereafter, the end surfaces of the legs are welded together by a longitudinal weld joint so that a hermetic and pressure resistant sealing of the recess is achieved.
A relatively deep recess must be provided at the edge portion of the control rod blade in order for the legs to obtain a sufficient length such that they can be rolled together. In a welded state, the free edge portion of the control rod blade gets a rounded shape and the absorber material must therefore be arranged at a not completely negligible distance from the end surface of the control rod blade. In order to increase the reactivity effect of the control rod blade, it would be desirable if the absorber material could be arranged closer to the end surface.
SE 519 331 shows a control rod blade comprising a plurality of channelswhich are arranged to receive an absorber material. The control rod blade has a free edge portion with a recesswhich comprises outlets for said channels in a bottom surface. A cover element is arranged to be attached in the recess in order to seal the edge portion. The cover element is substantially T-shaped and comprises a cover portionwhich seals the opening of the recess, and a support portionwhich is arranged to give the cover element a support against the bottom surface of the recess. The cover element is attached in the recess by means of two longitudinal weld joints. The support portion has here a widthwhich is less than the width of the outlets of the channels.
Thereby, the outlets of the channels are not sealed but they are connected to each other via the elongated passages, which are obtained on both sides of the support portion.
The elongated passages have here the task of allowing a pressure equalisation between the channels during operation and to allow a distribution of a protective gas to the root side of the weld joints during the welding operation. This cover element is intended to be used for control rod blades, which are provided with a solid absorber material such as, for example, absorber pins. If a powdered absorber material would be used, there is a risk that the powdered absorber material whirls up during the welding operation and is mixed into the weld joint.
By a profylax published in Research Disclosure 33925/92, a control rod blade is shown, which is provided with a concave recess at a free edge portion. In order to seal the recess, a cover element with a substantially corresponding concave shape is here applied in the recess. The cover element comprises a cover portion, which is arranged to seal the opening of the recess, and a support portion, which comprises a convex contact surface, which is arranged to get into abutment with a corresponding concave wall surface of the recess. The cover element is attached in the recess by means of two longitudinal weld joints. The contact surface of the support portion comprises a chamfered portion such that a longitudinal passage is formed between the support portion and the wall surface of the recess in a mounted state. This passage allows, during operation, a free flow of fission gases between the channels such that a pressure equalisation is obtained. The cover element comprises two elongated passages in connection with the weld joints. These elongated passages allow a distribution of a protective gas to the root side of the weld joints during the welding operation.
The cover element has here a shape such that the channels with absorber material are not connected to the elongated passages, which allow a distribution of a protective gas to the root side of the weld joints. Thereby, this control rod blade is suited to be provided with a powdered absorber material. However, the cover element has a relatively complicated shape, which makes it difficult to manufacture. Furthermore, the concave-shaped recess results in that the channels, at least locally, open out at a relatively large distance from the end surface of the control rod blade. The possibility of arranging the absorber material near the end surface of the control rod blade for achieving a high reactivity effect is here considerably reduced.